1. Field of the Invention
This invention relates to Hall-effect devices, and more particularly, to a control circuit that can be used in a Hall-effect switching device to control the output of the Hall-effect switching device in response to Hall effect.
2. Description of Related Art
The Hall effect is a well-known principle in the art of electronics and is widely used in the design of many various kinds of electronic devices, such as control devices in DC brushless motors. Since a Hall-effect control device can operate without physical contact with the moving part of the motor, it can allow the motor to be easily controlled without causing mechanical wear.
The principle of Hall effect is briefly described in the following with reference to FIG. 1. As shown, assume a magnetic field B is applied to a current-carrying semiconductor 2 in perpendicular to the flow direction of the current I flow through the semiconductor 2. Then, in the case of the semiconductor 2 being N-type, the positive carriers in the current I will be diverted upwards to the top of the semiconductor 2 while the negative carriers will be diverted downwards to the bottom, thus resulting in a potential difference V from top to bottom in the semiconductor 2; whereas in the case of the semiconductor 2 being P-type, the positive carriers will be diverted upwards to the top while the negative carriers will be diverted downwards to the bottom, thus resulting in a potential difference V from bottom to top in the semiconductor 2. This potential difference is customarily referred to as Hall-effect voltage. The magnitude of the Hall-effect voltage V is proportional to the intensity of the magnetic field B and the magnitude of the electric current I.
FIG. 2 is a schematic block diagram of a conventional Hall-effect switching device that uses a magnetic field to control the ON/OFF states thereof. As shown, the Hall-effect switching device includes a Hall-effect sensor 4, an amplifier 6, a hysteresis generator 8, a first control circuit 10, a second control circuit 12, a first bipolar junction transistor (BJT) IC1, and a second BJT IC2. In operation, the Hall-effect sensor 4 can generate an output signal indicative of the magnitude of a magnetic field B being applied to the switching device. The output signal from the Hall-effect sensor 4 is then amplified by the amplifier 6. In response to the amplified signal from the amplifier 6, the hysteresis generator 8 determines whether the applied magnetic field B is to cause a conducting state or (ON) a non-conducting state (OFF). The outputs from the hysteresis generator 8 are then transferred respectively under the control of the first and second control circuits 10, 12 to the first BJT IC1 and the second BJT IC2, causing the first BJT IC1 to produce a first output signal V.sub.out1 and the second BJT IC2 to produce a second output signal V.sub.out2, where V.sub.out1 and V.sub.out2 are complementary to each other.
FIG. 3 is a graph showing a hysteresis effect caused by the hysteresis generator 8 utilized in the Hall-effect switching device of FIG. 2. In this graph, the horizontal axis represents the magnitude of the applied magnetic field B (unit: Gauss), while the vertical axis represents the magnitude of the output voltage V.sub.o from the hysteresis generator 8 (unit: volt). As shown, the hysteresis plot has a positive threshold point B.sub.op and a negative threshold point B.sub.rp on the magnitude of the magnetic field B. When switching from non-conducting state (OFF) to conducting state (ON), the applied magnetic field B should be greater in magnitude than the positive threshold point B.sub.op to allow the output voltage V.sub.o to be turned into a high-voltage state, in which can then cause the switching device to be switched to the conducting state (ON). The conducting state (ON) will be maintained unless the applied magnetic field B is reduced to below the negative threshold point B.sub.rp where the output voltage V.sub.o will be turned into a low-voltage state (0 V) that can then cause the switching device to be switched to the non-conducting state (OFF).
One drawback to the foregoing Hall-effect switching device of FIG. 2, however, is that it has a poor symmetric relationship between the positive threshold point B.sub.op and the negative threshold point B.sub.rp. The difference of the absolute value of the positive threshold point B.sub.op and the absolute value of the negative threshold point B.sub.rp, i.e., the absolute quantity .parallel.B.sub.op .vertline.-.vertline.B.sub.rp .parallel., is typically greater than 30 G, which is considered too large to allow the switching device to have a high sensitivity in Hall-effect detection.
Moreover, the operating current, which flows through the semiconductor, is above 5 mA (milliampere), the switching device will require a starting voltage of above 4 V to operate, which causes the power consumption to be high. The sensitivity in Hall-effect detection can be raised by reducing the value of .parallel.B.sub.op .vertline.-.vertline.B.sub.rp .parallel.. However, this scheme requires a high working current to realize, which causes the power consumption to be further increased.
FIG. 4 is a schematic circuit diagram of a double-output transistor circuit that can generate the two complementary output signals V.sub.out1 and V.sub.out2 shown in FIG. 2 in response to the one single output signal from the hysteresis generator 8 (here denoted by V.sub.t). The circuit of FIG. 4 represents a realization of the first control circuit 10, the second control circuit 12, the first BJT IC1, and the second BJT IC2 in the Hall-effect switching device of FIG. 2.
In operation, when the input signal V.sub.t is switched into high-voltage state, it will cause the first BJT IC1 to be switched into conducting state, thereby causing the second output signal V.sub.out2 to take on the ground voltage (i.e., a low-voltage logic state). Meanwhile, it also causes the gate voltage of the second BJT IC2 to take on the ground voltage, thereby causing the second BJT IC2 to be switched into non-conducting state, thus allowing the first output signal V.sub.out1 to take on the system voltage V.sub.dd (i.e., a high-voltage logic state).
On the other hand, when the input signal V.sub.t is switched into low-voltage state, it will cause the first BJT IC1 to be switched into non-conducting state, thereby causing the second output signal V.sub.out2 to take on the system voltage V.sub.dd (i.e., a high-voltage logic state). Meanwhile, it also causes the gate voltage of the second BJT IC2 to take on the system voltage V.sub.dd, thereby causing the second BJT IC2 to be switched into conducting state, thus allowing the first output signal V.sub.out1 to take on the ground voltage (i.e., a low-voltage logic state).
One drawback to the foregoing double-output transistor circuit, however, is that the generation of the two output signals V.sub.out1 and V.sub.out2 requires two transistors, i.e., the first BJT IC1 and the second BJT IC2, to realize, which not only causes the overall power consumption by the switching device to be high, but also requires a large circuit layout area to implement.